Battle of Tannenberg

The Battle of Tannenberg, also known as the Second Battle of Tannenberg, was fought between Russia and Germany between 23 and 30 August 1914, the first month of World War I. The battle resulted in the almost complete destruction of the Russian Second Army and the suicide of its commanding general, Alexander Samsonov. A series of follow-up battles destroyed most of the First Army as well and kept the Russians off balance until the spring of 1915.

The battle is particularly notable for fast rail movements by the German Eighth Army, enabling them to concentrate against each of the two Russian armies in turn, first delaying the First Army and then destroying the Second before once again turning on the First days later. It is also notable for the failure of the Russians to encode their radio messages, broadcasting their daily marching orders in the clear, which allowed the Germans to make their movements with the confidence they would not be flanked.

Source: Wikipedia